For the purpose of machining a workpiece, a wire cut electric discharge machine intermittently generates a spark or discharge between the workpiece and a wire electrode (hereinbelow, referred to simply as a wire) running along a wire running passage, while displacing the wire with respect to the workpiece. A wire feed device to be mounted on the electric discharge machine and for running the wire, typically comprises a wire bobbin, a guide roller for guiding the wire, a tension roller operative to apply a tension to the wire, and a feed roller for the drive of the wire, in which the wire is tensioned-along the wire running passage extending through the above-described rollers, and then the feed roller is rotationally driven to deliver the subsequent wire from the wire bobbin.
During the delivery of the wire, a wire delivery position lying on the wire bobbin is displaced in the width direction of the bobbin, and the direction in which the wire delivery position is displaced is reversed when the outermost wire layer is replaced by the subsequent wire layer. With the displacement of the wire delivery position, there varies an angle (hereinafter, referred to as a wire delivery angle) defined by an imaginary line extending through the wire delivery position and orthogonally to the axis of the wire bobbin, and a line obtained by projecting the wire let out from the wire bobbin onto an imaginary plane extending through the wire delivery position.
In case the wire is delivered diagonally from the wire bobbin toward the guide roller with a larger wire delivery angle, the delivered wire possibly runs onto the adjacent wire constituting a wire layer to which that wire is to belong. On the contrary, with the displacement of the wire delivery position, the wire which ran onto the adjacent wire possibly returns to the ordinary winding position. Such running onto the adjacent wire and or return to its ordinary position of the wire (hereinafter, referred to as a "jump" of the wire), may lead to a fluctuation in the wire tension, to degrade the machining accuracy, in particular, the surface roughness, which brings about a deterioration in quality of machined product.
Incidentally, it is possible to reduce the wire delivery angle which would otherwise cause a "jump", by enlarging the interval between the wire bobbin and the guide roller. The provision of a greater interval between the wire bobbin and the guide roller enough to prevent the occurrence of "jump" leads to an excessive enlargement in size of the electric discharge machine, which is impractical.